Guest-adaptive molecular sensing in a dynamic 3D covalent organic framework

Molecular recognition is an attractive approach to designing sensitive and selective sensors for volatile organic compounds (VOCs). Although organic macrocycles and cages have been well-developed for recognising organics by their adaptive pockets in liquids, porous solids for gas detection require a deliberate design balancing adaptability and robustness. Here we report a dynamic 3D covalent organic framework (dynaCOF) constructed from an environmentally sensitive fluorophore that can undergo concerted and adaptive structural transitions upon adsorption of gas and vapours. The COF is capable of rapid and reliable detection of various VOCs, even for non-polar hydrocarbon gas under humid conditions. The adaptive guest inclusion amplifies the host-guest interactions and facilitates the differentiation of organic vapours by their polarity and sizes/shapes, and the covalently linked 3D interwoven networks ensure the robustness and coherency of the materials. The present result paves the way for multiplex fluorescence sensing of various VOCs with molecular-specific responses.

The observed reflection conditions can be summarized as hk0: h + k = 2n, 00l: l = 2n, which are consistent with the PXRD pattern.

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The central carbon atom positions of the tetrahedral building block can be located from this potential map. Compared to the constructed structure model, the origin of the unit cell in this potential map shifts 1/2 alone the a axis.
To obtain a reasonable framework, the possible structure naturally follows the dia-cN topology. Three possible candidate structure models with the interpenetration of 6fold, 10-fold, and 14-fold were exanimated (Supplementary Figure 20) 4 . The result shows that the length between two adjacent central carbon atoms of the 6-fold structural model is too short while the 14-fold one is very long. At the same time, the 14-fold interpenetrated structure model is too crowded which indicates that the structure is not geometrically robust. Therefore, the 10-fold interpenetrated structural model was determined to be a proper choice. During the construction of 10-fold interpenetrated structure model, we found that the adjacent linear linking units are still a little bit close to each other if no fragment distortion was involved. This indicates that the structure needs to be distorted a little bit to ensure a more reasonable geometry. Then the generated model was further refined by Rietveld refinement against PXRD intensity.
The final structure of dynaCOF-330 is identified to adopt a 10-fold interpenetrated dia topology with Pnn2 space group.       The in-situ PXRD under varied temperatures illustrates the structure changed slightly upon temperature variation.

Supplementary Section 5. In-situ fluorescence spectroscopy for dynamic multi-component gas sensing.
A special customized relative humidity and gas partial pressure controller was prepared as shown in Supplementary Figure 45. Two mass flow controllers (MFCs) with different controlling ranges (100 sccm, and 5 sccm for low partial pressure less than 5%) were connected to n-C4H10 cylinder. Two MFCs were connected to N2 cylinder as a purge gas to activate the sample or balanced gas mixed with n-C4H10. The N2 and n-C4H10 mixture was then passed through a water bottle with saturated MgCl2 to humidify the working gas with 53% relative humidity. A tee valve was used to switch the wet working gas to dry gas. All the stainless-steel valves and joints were purchased from Shanghai X-tec Fluid Technology Co, Ltd, and the MFCs were purchased from Alicat Scientific (A Halma company). The film sample was prepared by dropping a slurry of dynaCOF-330 (10 mg dispersed in 3 mL acetone) in the washed glass slide (1 cm × 3 cm) to measure its fluorescence response for n-butane gas. Before measurement, the COF film was dried under vacuum at room temperature for 5 hours to ensure the guest was entirely removed. pressure, a Nose-Hoover chains method 12 was applied during the whole process. For the result, the dihedral analysis was based on the production run. We chose the frame structure at 0.5 ps as the first initial frame. The first plane is composed of the three carbon atoms. Every 0.5 ps, we chose a shot from the trajectory and the second plane consists of the same three atoms. The angle between the two planes is the dihedral.
When the dihedral is beyond 90 degrees, it means the whole fragment is upside down.
For a better understanding of the flexibility of the COF, we rule the dihedral is 180 degrees minus angle when the dihedral is beyond 90 degrees.